A vacuum-assisted, countergravity casting process using a gas permeable mold is described in such prior patents as the Chandley et al U.S. Pat. Nos. 4,340,108 issued July 20, 1982 and 4,606,396 issued Aug. 19, 1986. That countergravity casting process involves providing a mold having an expendable porous, gas permeable upper mold member (cope) and an expendable lower mold member (drag) engaged together at a parting plane, sealing the bottom lip of a peripheral wall of a vacuum chamber to the mold such that the vacuum chamber confronts the gas permeable upper mold member, immersing the bottom side of the lower mold member in an underlying pool of melt, and evacuating the vacuum chamber to draw the melt upwardly through one or more melt inlet passages in the lower mold member and into one or more mold cavities formed between the upper and lower mold members.
Typically, this process fills the mold cavities with the melt in a relatively short time, such as approximately 2-3 seconds. However, the mold must remain immersed in the melt until at least the melt in the inlet passages of the mold drag freezes off (solidifies). For example, the mold typically must remain immersed in the melt pool on the order of 15 to 50 seconds after filling of the mold cavities to freeze off (solidify) the melt in the inlet passages. In the event the mold is withdrawn from the melt pool prior to freezing of the melt in the inlet passages, the melt in the inlet passages as well as in the mold cavities can flow downwardly out of the mold and result in a defective casting which must be scrapped. The time required to freeze off the melt inlet passages has thus limited the rate of production of castings using this process.
The Greanias et al U.S. Pat. No. 4,862,945 issued Sept. 5, 1989 illustrates use of a ball check valve in a melt inlet passage of a drag member of a vacuum-assisted, countergravity casting mold assembly to prevent backflow of the melt from the mold cavity when the drag member is withdrawn from the underlying melt pool after filling of the mold cavity and before solidification of the melt in the inlet passages. The drag member is immersed in the melt pool only long enough to fill the mold cavities and thereby reduces the overall casting cycle time to improve the rate of production of castings.
The ball check valve is disposed on a valve seat formed on the drag member so as to lifted off the seat by the upwardly flowing melt during casting to permit filling of the mold cavity with the melt and then is forced downwardly by the head (fluid pressure) of the melt thereabove into sealing engagement with the valve seat after casting as the drag member and the melt pool are disengaged so as to prevent the melt in the mold cavities from draining downwardly out of the mold. Thus, movement of the ball check valve between open and closed positions relative to the valve seat is effected by the melt itself during the casting process.
It is an object of the invention to provide an improved apparatus for the vacuum-assisted, countergravity casting of a melt in reduced casting cycle times by virtue of including a valve movable on a mold relative to a melt inlet of a mold cavity by valve moving means carried on a vacuum box confronting the mold and actuable after the mold cavity is filled with the melt to move the valve to a closed position relative to the melt inlet so as to allow disengagement of the mold and an underlying source of the melt prior to solidification of the melt in the melt inlet without backflow of the melt from the mold cavity.